A radar system usually includes an antenna, a radar transceiver, and a radar display. The radar transceiver sends pulse signals to the radar antenna. The pulse signals are then transmitted by the antenna as an electromagnetic wave. The electromagnetic wave is reflected by objects within the range of the radar system and is returned to the antenna as received signals. The antenna also serves to couple these received signals to the radar transceiver. The radar transceiver operates on the received signals to develop a video signal suitable for presentation to the radar display. Typically, the radar transceiver also provides synchronization for the video signal in the form of a trigger signal. Most modern radars develop digital data signals to present to the radar display. The digital data signals may include various status information such as current antenna position, pulse repetition frequency, and data sychronization information. The radar display operates on the video, trigger, and data signals to create a visual presentation of objects within the range of the radar together with the status information.
It is thus necessary to send video, trigger, and data signals from the radar transceiver to the radar display. Sometimes the radar transceiver is located away from the radar display, such as in the case of a marine radar where the antenna and transceiver are typically located at the top of a ship's mast and the display is located inside the ship's bridge. This situation also occurs in radars adapted for harbor surveillance as it is common to have several remote radar antennas linked via radio to a control center having several radar displays located within it. In such instances it is desirable to minimize the number of transceiver to radar display links.
Some prior systems have solved this problem using a technique known as time multiplexing. In this technique, time periods are pre-assigned for transmission of each of the trigger, video, and data signals over a single link from the transceiver (originating end) to the display (receiving end). Because the time duration of the video signal varies as the range and/or pulse repetition frequency setting of the transceiver, this approach invariably requires the receiving end to have prior knowledge of the time duration and location of the trigger, video and data. To achieve this requires complex timing circuits at the demultiplexer and either prior knowledge of a change in timing or a slow response to changes made by the originating end. This results in not using all the available time on the link. An alternate approach for maximizing the use of a link is frequency multiplexing, where each of the signals to be transmitted is multiplexed onto a different radio frequency carrier signal. This allows more efficient time use of the link by enabling parallel transmission. However, frequency multiplexing is typically more expensive to implement than time multiplexing.
Many television systems solve the problem of sending both trigger and video signals on a single link by sending a trigger pulse before the video signal, with the video signal having a maximum voltage level less than the minimum high trigger voltage. Other television systems send a trigger pulse having an inverted polarity. This technique can be used to scramble the the video signal so that only displays capable of operating with an inverted trigger signal may use the video signal.